I found, when I got my little cards, that size matters. The little cards can't be mailed. Sure, you can put them in a larger envelope to mail them, and I've actually successfully mailed one, but it got delivered with a note warning that the size was non-standard. And people will want to, you know, mail cards...
The Post Office says that the minimum size for mailing anything is 3½" x 5". My little cards are 2½" x 3½".
I feel like Goldilocks, and I've tried Papa Bear's bed, and Baby Bear's bed... how about just right?
(I don't think the little cards are a mistake... they do have their uses. But I need to offer something that's mailable and small.)
That's the smallest size there is, and it's called A-1. Small and mailable. That works, and so I think I'll order up a bunch of those.